Walrog can randomly spawn in dungeons with water and can be forced to spawn by killing both the dragon and naga bosses in the Eastern water dungeon.

Kryl'fejn can be found be entering the Dark Crypt(World map event, 24+) and letting Melinda be sacrificed.

Shassiy'keijn is summoned through a random event. This even has cultists standing next to monoliths, and killing one will star the countdown to her arrival. This event can be found in Daikara, Mark of the Spellblaze, Elven Ruins, Reknor, and the Infinite Dungeon and has on average a ~72% chance to be found on a run (ignoring ID).

While a Doombringer, and using a race that starts out in the off-planet demon area (i.e. not undead/dwarf or other overriding start), you will be abducted while using the rod of recall at roughly level 20. completing this level/event will unlock Demonologist.

Note: if you you just press '5' and wait for your future self to win the fight you will miss out on an artifact bow that goes well with Temporal Warden. You can bring your future self down to half health, defeat the optional boss and then get defeated by your future self if you want said artifact bow.

Counterstrike an enemy from a distance of 5 or more squares away. Can be achieved by blocking a ranged attack then shooting the attacker, blocking up close and quickly getting far enough away to then shoot, or many other methods. Note, despite being previously mentioned here using Rush will not work as you are no longer 5+ squares away when you strike.

Inflict over 1,000,000 fire damage, having already unlocked Archmages. It counts all damage done by the player, whatever the source -- spells, weapons, etc. -- and it counts over multiple games. You can check the progress of this unlock under the Pyromancer achievement.

Inflict over 1,000,000 cold damage, having already unlocked Archmages. It counts all damage done by the player, over multiple games. You can check the progress of this unlock under the Cryomancer achievement.

Items

After picking up the chest, each new character will start with such a chest.

Other

Zigur Betrayal

Read the signpost at the entrance to Zigur to enable all new qualifying characters (i.e. those without runes or magic talents) to betray escorts to Zigur.

Redhead cosmetic option

Save Melinda in the Dark Crypt to unlock the redhead birth option for humans, elves, halflings and dwarves. You must be a donator to use this option once unlocked.

Female Dwarf Beards cosmetic option

Defeat The Mouth in The Deep Bellow as a female dwarf to unlock the bearded female dwarf. You must be a donator to use this option once unlocked.

Insane Difficulty

Win the game in Nightmare difficulty to unlock the even harder Insane difficulty. You have to win the main campaign, winning the arena mode in Nightmare difficulty will not unlock Insane. However, once you have Insane difficulty unlocked, it will be available for all other game modes as well.

Madness Difficulty

Win the game in Insane difficulty to unlock the uber-hard Madness difficulty. You have to win the main campaign, winning the arena mode in Insane difficulty will not unlock Madness. However, once you have Insane difficulty unlocked, it will be available for all other game modes as well.

Bikini/Mankini cosmetic option

Complete Melinda, lucky girl to unlock the Bikini and Mankini birth option. You must be a donator to use this option once unlocked.

Demon Horns cosmetic option

Use the demonologist ability demon horns to kill an enemy. You must be a donator to use this option once unlocked.

Red Skin cosmetic option

Have five demon seeds equipped at once as a Demonologist. You must be a donator to use this option once unlocked.